Encyclopedia
Please note this is an old version of this entry, which may differ significantly from the current revision.
We need sustainable solutions to avoid global warming and climate change. Homogeneous catalysis might play a fundamental role for this. Pincer-type complexes are promising in terms of stability, selectivity, efficiency and the use of mild reaction conditions. Pincer complexes have been used in many sustainable chemical reactions, for example hydrogen release and upconversion of CO2, N2, and biomass.
- pincer complexes
- sustainability
- biomass valorization
- hydrogen
- carbon dioxide valorization
- nitrogen fixation
1. Introduction
Pincer complexes are highly promising catalysts for numerous sustainable processes. High catalytic activity under mild reaction conditions, low catalyst loading, and high selectivity are the general main advantages important for sustainable reactions as guided by the green chemistry guidelines [1]. Increased robustness because of the pincer stabilization results in increasinly stable homogeneous catalytic systems [2,3,4].
The pincer complex, of the PCP type, was synthetized by Shaw in 1976 [7]. Numeous pincer complexes have since been synthetized as well as used in catalysis [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,40,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129]. Examples are [130,131,132], hydroamination [133,134,135], hydrocarboxylation [136], hydrovinylation [137], aminomethylation [138], alkane dehydrogenation [139,140,141,142,143,144], alkane metathetis [145], amine N-formylation [146,147], secondary alcohol C-alkylation [148,149], ketone α-alkylation [150,151], and amine alkylation [152,153,154,155,156], aniline alkylation [157], vicinal diol deoxydehydration [106,107,158,159,160,161], and water splitting [14,162,163,164,165,166,167,168,169].
2. Dehydrogenation Reactions
2.1. Early Works
In the 1960s, Charman showed the first example of acceptorless alcohol dehydrogenation by homogeneous catalysis [198]. In the mid-1970s, Robinson demonstrated the dehydrogenation of isopropanol, 1-butanol, ethanol, methanol, and glycerol [199,200,201,202].
2.2.1. Methanol Dehydrogenation
In 2017, Beller proposed a manganese catalyst for aqueous methanol dehydrogenation [346]. Ethanol, paraformaldehyde, and formic acid were also dehydrogenated. Beller also showed that an iridium-PNP catalyst promotes methanol dehydrogenation under mild conditions [347]. In 2019, Beller improved the activity of Ru-PNP catalysts for methanol dehydrogenation using a bi-catalytic system [335].
2.2.2. Formic Acid Dehydrogenation
Formic acid as hydrogen carrier and storage system has been reviewed in many works [299,300,350,351,352,353,354,355,356,357,358,359]. For example, Milstein showed iron pincer complexes with a lutidine moiety as catalysts for hydrogenation of ketones [385,386], CO2 hydrogenation to formate [387], and formic acid dehydrogenation [388].
3. Hydrogenation Reactions
Pincer complexes have been used in the (transfer) hydrogenation of many substrates, such as ketones [385,457,458,459,460,461,462], esters [40,179,220,386,400,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477], aldehydes [478,479,480], amides [67,481,482,483,484,485], and imines [486,487].
CO2 is potentially a C1 building block, increasing sustainability [492,493,494]. CO2 capture from the atmosphere or from localized emission sources have been studied [495,496,497,498,499,500,501,502,503,504]. CO2 is subsequently stored ior utilized in synthesis of value-added products [505,506,507,508,509,510]. The industry uses several million tons of CO2 for producing e.g. urea, salicylic acid, cyclic carbonates, and polypropylenecarbonate [511,512,513].
Catalytic hydrogenation of CO2 has gained attention for storing green hydrogen with seminal works by Asinger [514], Leitner [515,516], Noyori [517], and Olah [301,518,519,520]. The synthesis of methanol from CO2 is typically carried out at high temperatures and pressures by heterogeneous catalysts such as Cu/ZnO/Al2O3 [521,522,523,524]. Homogeneous catalytic systems have been usedfor the hydrogenation of CO2 into green fuels [213,214,517,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544].
3.1. CO2 Hydrogenation
3.1.2. CO2 Hydrogenation to Methanol
In 2016, Himeda and Laurenczy reported an iridium complex to catalyse bicarbonate hydrogenation to formate as well as formic acid dehydrogenation [584], and for the production of methanol from CO2 in the presence of sulfuric acid [585]. In 2015, Sanford employed a ruthenium-based catalytic hydrogenation of CO2 to methanol with dimethylamine as capturing agent [586]. The same year, Milstein developed indirect CO2 hydrogenation by prior CO2 capture by amino alcohols followed by hydrogenation of the resulting oxazolidinone to form MeOH [587]. The year after, Olah and Prakash demonstrated a catalytic system for a one-pot CO2 capture as well as conversion to methanol employing polyamine and Ru-MACHO-BH [589].
This entry is adapted from the peer-reviewed paper 10.3390/catal10070773
This entry is offline, you can click here to edit this entry!
